1. Technical Field of the Invention
The present invention relates to a piston ring for internal combustion engines, comprising a steel or cast iron base material; and a coating of chromium nitride deposited by a physical vapor deposition process on a sliding surface of the piston ring.
2. Description of the Related Art
The use of a chromium nitride film with typical crystal structure of CrN phase on the outer peripheral surface of a piston ring provides high wear resistance for this component. This film is widely used in modern engines, last developments, with high mechanical and thermal loads. Such coating is obtained by physical metallic vapor deposition usually generated by a source of cathodic arc.
However, in some recent engines extremely high loaded or in engines with a high level of combustion pressure, as the ones designed for the next decade, this ceramic coating of chromium nitride presents a intrinsic fragility which takes to the appearance of micro-cracks on its surface. These micro-cracks propagate and their connection takes to a loss of small pieces of the coating, generating damages on its surface and, in some cases, scratches on the engine liners. Not only the scratches but also the damages are usually considered functional failure of the piston rings.
The improvement of resistance to micro-crack generation, which antecedes the loss of material, and the improvement of resistance to scratches on the engine liners is presented in specialized literature through the generation of a chromium nitride film having additions of oxygen, carbon and boron, in solid solution form in the crystalline coating of the CrN phase. The addition of these elements is presented as intrinsically responsible for the resistance increase of micro-cracks.
The change in the crystal structure of a physical vapor deposited film with the addition of small quantities of other reacting gases leads to a change of the properties of this coating, and this is a classic technique established in the literature, for example by Mattox in the Handbook of Physical Vapor Deposition (PVD) Processing, Noyes Ed, page 486. Literature mentions that physical vapor deposited films present columnar crystal morphology with the preferential growth of specific crystal orientations parallel to the surface of the film. The preferred orientation of a specific crystal plane can change the properties of the film and depends on the deposition parameters, such as substrate temperature, addition of reactive gases and ion bombardment.
U.S. Pat. No. 5,743,536 mentions that the crystals of CrN might be predominantly oriented with the (111) dense planes parallel to the surface and that with such orientation there is an improvement of the film peeling. Such predominant orientation was obtained by the manipulation of the deposition parameters, with no external addition of other doping elements to the nitrogen reacting gas. However, it is not mentioned whether or not such orientation is the only one observed, i.e. if preferred orientation means that 100% of the crystal parallel to the surface are of grains with (111) crystal orientation, or in which proportion other crystal orientations are allowed.
Besides, this relatively broad definition regarding the relative content of other crystal planes not so dense as (111) planes, the CrN coating produced with the process described in U.S. Pat. No. 5,743,536 has a relatively low Vickers hardness of 600 to 1000 HV, which would jeopardize the film wear resistance making it not applicable to the recent highly loaded engines.
UK Patent No. GB 2,276,176 describes the doping of CrN with 3 to 20 weight percent of oxygen or, alternatively, the doping of CrN with 2 to 11 weight percent of carbon. The main objective of this was to provide a coating with higher wear and scuffing resistance. The possible crystalline change of CrN coating by the addition of oxygen was not brought into discussion or included in the protection claims of the referred patent document. An X-ray diffraction chart presented in FIG. 6, page 6, only showed that CrN structure has a dominant (200) plane after doping with 10 weight percent of oxygen.
U.S. Pat. No. 6,149,162 describes the deposition of a chromium nitride film with CrN crystal structure having 0.5 to 20 weight percent of oxygen and presenting a microstructure with dominant or preferential orientation of crystal plane (200) parallel to the coating surface. It is mentioned in this patent that a crystal structure of CrN with a preferred orientation of (111) crystal plane is brittle in comparison to the preferred orientation of the (200) crystal plane parallel to the coating surface. In this patent, there was a connection of the oxygen addition to the crystalline coating and the consequent alteration of its crystal structure, giving rise to an improvement of the coating resistance concerning the generation of micro-cracks. However, the definition of preferred orientation is very broad, not allowing the definition of a predominant level, whether or not it means 100% or whether or not allowed a residual level of planes (111) described as brittle is allowed. Likewise, the definition of preferred orientation does not impose criticality to distinct levels of intensity ratio of crystal planes (111) and (200) in the CrN coating. Furthermore, U.S. Pat. No. 6,149,162 depicts that neither a general preferred orientation of less dense (200) crystal planes parallel to the surface nor an addition of 0.5 to 20% of oxygen to a CrN coating with preferred (200) orientation would improve the coating spalling resistance.
Furthermore, it is known that chromium nitride of crystal structure or CrN phase appears in its form established as standardized in nature with intensity ratios between crystal planes (111) and (200), measured by x-ray diffraction, of approximately 0.80. This information is registered on the JCPDS card no. 11-65 (Joint Committee of Powder Diffraction Standards) of the database of composts of ICDD—International Centre for Diffraction Data. Therefore, the presence of a predominant plane must be compared to this intensity ratio of crystal planes of the CrN phase of reference.
International Patent Application No. WO 2006/131801 discloses that the ratio of crystal planes (111) and (200) is limited to a maximum of 0.70 within the trivial deposition parameters and within a proposed oxygen content of 1.0 to 7.0 wt %.
In the above related art technologies, the chromium nitride coating with CrN crystal structure may be configured with a (111) crystal plane preferred orientation, presenting low hardness associated with the deposition parameters used for the obtainment of this orientation, or a (200) crystal plane preferred orientation parallel to the surface, obtained with the addition of a wide range of oxygen. In neither of the above technologies there is a suggestion of a preferred balanced orientation, which allows the presence of both a specific ratio between dense (111) crystal planes and the open structure (200) crystal planes.